I had to re-install OS 10.6.4 and then do all the updates except the last Security Update, so I was eventually back up to 10.6.8. Took about 4 hours. Next time I'll have a backup to use instead. It might take less time...

I installed the corrected Security Update, and everything appears to be working properly now. Freehand "Opens", "Saves" and "Saves As" like it should. Problem solved! I assume other Rosetta-dependant apps are also workng for those people using them.